1. Field of the Invention
The present invention relates to a printed board which supports circuit devices including transistors, ICs (Integrated Circuits), LSI (Large Scale Integration) circuits, etc.
2. Description of the Related Art
Printed boards support circuit devices including transistors, ICs, LSI circuits, etc. suffer conductive or radiant electromagnetic noise due to high-frequency currents generated by the circuit devices and circulating currents known as a common-mode signal flowing through parasitic capacitances and parasitic mutual inductances which circuits on the printed boards have. When electromagnetic noise is generated, it tends to cause an error in electronic units which incorporate such printed boards or other electronic units.
To solve the above problem, one conventional printed board has employed circuit interconnections as shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1, the conventional printed board supports circuit devices 101, 102, 103 for generating high-frequency currents whose magnitudes are different from each other, the circuit devices 101, 102, 103 being connected parallel to each other between a power supply line 104 and a ground line 105. The circuit device 101 generates a high-frequency current whose magnitude is the greatest, the circuit device 102 generates a high-frequency current whose magnitude is the second greatest, and the circuit device 103 generates a high-frequency current whose magnitude is the third greatest. The power supply line 104 is formed in a power supply layer (not shown) of the printed board, and the ground line 105 in a ground layer (not shown) of the printed board. The power supply layer is disposed as a planar layer which comprises an electrically conductive layer formed over the entire surface of the printed board.
The printed board is supplied with electric energy from a power supply which is connected to a power supply terminal 104a of the power supply line 104 and a power supply ground terminal 105a of the ground line 105. A decoupling capacitor 106 is connected between the power supply line 104 and the ground line 105 near the power supply terminal 104a and the power supply ground terminal 105a.
Decoupling capacitors 107a, 107b, 107c, which have respective capacitances commensurate with the magnitudes of the high-frequency currents generated by the respective circuit devices 101, 102, 103, are connected across the respective circuit devices 101, 102, 103 between power supply terminals 101a, 102a, 103a and ground terminals 101b, 102b, 103a of the respective circuit devices 101, 102, 103. The decoupling capacitor 107a has an electrostatic capacitance whose magnitude is the greatest, the decoupling capacitor 107b has an electrostatic capacitance whose magnitude is the second greatest, and the decoupling capacitor 107c has an electrostatic capacitance whose magnitude is the third greatest. Since the impedances of the decoupling capacitors 107a, 107b, 107c are inversely proportional to the magnitudes of their electrostatic capacitances, the decoupling capacitor 107c has an impedance whose magnitude is the greatest, the decoupling capacitor 107b has an impedance whose magnitude is the second greatest, and the decoupling capacitor 107a has an impedance whose magnitude is the third greatest.
The high-frequency currents generated by the circuit devices 101, 102, 103 flow respectively through the decoupling capacitors 107a, 107b, 107c to the ground line 105, so that the voltages at the power supply terminals 101a, 102a, 103a of the circuit devices 101, 102, 103 are prevented from varying.
In as much as the power supply layer of the printed board is formed as a planar layer over the entire surface of the printed board, the resistance, i.e., the impedance, of the power supply line 104 is small. Consequently, the voltage at the power supply layer is also prevented from varying even when high-frequency currents flow into the power supply line 104.
The impedance of the power supply layer is smaller than the impedances of the decoupling capacitors 107a, 107b, 107c. Therefore, the high-frequency currents generated by the circuit devices 101, 102, 103 may not flow through the decoupling capacitors 107a, 107b, 107c, but may flow through the power supply layer into other circuit devices or decoupling capacitors which have smaller impedances or larger electrostatic capacitances.
Since the high-frequency currents flowing through the decoupling capacitors 107a, 107b, 107c are very complicated, it is difficult to accurately grasp electrostatic capacitances that are required by the respective decoupling capacitors 107a, 107b, 107c. In addition, decoupling capacitors have different impedance vs. frequency characteristics depending on the electrostatic capacitance even if they are of the same type. For example, the impedance of a decoupling capacitor having a smaller electrostatic capacitance tends to be smaller than the impedance of a decoupling capacitor having a larger electrostatic capacitance at frequencies higher than a certain frequency. Consequently, high-frequency currents whose frequencies reside in a wide frequency band flow into and out of the decoupling capacitors. For this reason, printed boards need to be fitted with many capacitors whose electrostatic capacitances are much greater than the electrostatic capacitances that are theoretically required.
When a high-frequency current flows into the planar power supply layer, the high-frequency current may act as a loop current in the power supply layer, or a common-mode high-frequency current may flow into a cable connected to the printed board, resulting in the radiation of electromagnetic noise. If a high-frequency current generated by a circuit device does not flow into a decoupling capacitor connected thereacross, but flows into another path, then the impedance of the path is increased, causing a large voltage variation in another circuit device, whose stable operation will adversely be affected.
An electronic unit which comprises a plurality of such printed boards is housed in a metallic casing for preventing electromagnetic noise from leaking out. Cables connected to the electronic unit and extending out of the casing are fitted with common-mode coils and cores for suppressing the conduction of electromagnetic noise. However, because of the presence of openings defined in the casing which receive control knobs and switches of the electronic unit, it is difficult to completely reject the leakage of electromagnetic noise out of the casing.
Formation of inductors on printed boards with printed interconnections on the printed boards is disclosed in Japanese laid-open patent publications Nos. 300593/88, 25497/89, 273699/91, and 293416/96.
Specifically, Japanese laid-open patent publication No. 300593/88, discloses laminating a magnetic sheet to a coil pattern to form an inductor for thereby increasing the inductance. According to Japanese laid-open patent publication No. 25497/89, a magnetic layer is disposed on a first conductor layer, and covered with a second conductor layer, making up an inductive device.
Japanese laid-open patent publications Nos. 273699/91 and 293416/96 show the formation of a coil-shaped inductor by successively connecting printed interconnections on two superposed conductor layers via through holes.
All the above disclosed conventional arrangements employ printed interconnections on the printed board to form an inductor on the printed board. Nothing in the disclosed printed boards refers to reducing electromagnetic noise radiated from a printed circuit having decoupling circuits as described with reference to FIG. 1.